This invention relates to a support apparatus for the primary mirror of a reflecting telescope, and to a mirror support system.
Large optical reflecting telescopes are essential tools of astronomical research. For useful observations, the primary mirror of a reflecting telescope must maintain a surface figure accurate to within 1/6 the wavelength of light, or approximately 100 nanometers, despite the action of forces tending to distort the mirror's shape. The main distorting force is the mirror's own weight, which causes the mirror to sag to varying degree and in varying directions depending on the telescope's attitude.
The traditional means of mirror support is the astatic lever, which has been widely employed since its invention by Lassel in 1841. This is a passive supporting means that uses a counterweighted lever to provide a supporting force that balances the weight of the mirror correctly regardless of the telescope attitude. Since it is relevant to the present invention, the theory of the astatic lever will be briefly described below.
FIG. 1 shows the astatic lever concept. The mirror 1 is linked to a lever 2 in such a way that the lever 2 supports the mirror 1 in the axial direction, parallel to the optic axis Z.sub.OA of the telescope. A counterweight 3 is affixed to the end of the lever 2 distant from the mirror. Let W.sub.1 be the weight of the mirror 1, let W.sub.A be the vector component of this weight in the axial direction, and let .theta. be the attitude of the telescope axis with respect to the horizon. Then: EQU W.sub.A =W.sub.1 sin.theta.
Similarly, let W.sub.2 be the weight of the counterweight 3, let l.sub.1 be the length of the lever 2 from its pivot point to the end near the mirror 1, let l.sub.2 be the length from the pivot point to the counterweight 3, and let F.sub.A be the supporting force provided by the lever 2 in the axial direction. Like W.sub.A, the quantity F.sub.A obeys a sine law: EQU F.sub.A =(W.sub.2 sin.theta.).times.(l.sub.2 /l.sub.1)
The condition for equality of W.sub.A and F.sub.A is: EQU W.sub.1 =W.sub.2 (l.sub.2 /l.sub.1)
Thus if the length l.sub.2 from the pivot point to the counterweight 3 is properly adjusted, the lever 2 will provide a supporting force that exactly compensates for the mirror's weight, regardless of the telescope attitude.
FIG. 2 shows a similar astatic lever mounted to provide radial support. Let W.sub.R be the vector component of the weight of the mirror 1 in the radial direction, F.sub.R be the supporting force provided by the lever 2 in the radial direction, W.sub.3 be the weight of the counterweight 3, and l.sub.3 and l.sub.4 be the lengths of the lever 2 from its pivot point to its end near the mirror 1 and to the counterweight 3, respectively. In this case W.sub.R and F.sub.R both obey a cosine law: EQU W.sub.R =W.sub.1 cos.theta. EQU F.sub.R =(W.sub.3 cos.theta.).times.(l.sub.4 /l.sub.3)
The condition for equality of W.sub.R and F.sub.R is: EQU W.sub.1 =W.sub.3 (l.sub.4 /l.sub.3)
If the length l.sub.4 is properly adjusted, once again the correct supporting force will be applied regardless of the telescope attitude.
The primary mirror of a reflecting telescope is mounted in a structure called a mirror cell, which must support the mirror in both the axial and radial directions. In a large telescope, the primary mirror is generally "floated" on a plurality of astatic levers that provide radial support at the mirror periphery and axial support at interior points, as illustrated in FIG. 3. The mirror cell 4 in FIG. 3 also has fixed supports 5 which support the mirror rigidly at, for example, three points (only two of which are shown in the drawing). Support apparatus of the type illustrated in FIG. 3 was used in almost all large reflecting telescopes built before 1970.
Although simple and elegant, the astatic lever alone is inadequate for the largest telescopes being designed at present, which will have primary mirros with diameters on the order of 8 meters. To reduce the cost of the mirror blank and the time require for annealing, such mirrors will have a meniscus shape with a high aspect ratio; i.e. they will be extremely thin in relation to their diameter. This aggravates the problem of mirror sag.
The maximum allowable thickness for a mirror 8 meters in diameter is considered to be about 20 centimeters. To maintain an accurate surface figure, a mirror of these dimensions must receive support in both the axial and radial directions at a large number of points. In particular, it must recieve radial support at interior points as well as around its periphery. If such a mirror is supported at, say, 400 points, then the required supporting force at each point is approximately 50 kilograms in the combined radial and axial directions. The maximum allowable error is .+-.15 grams in the axial direction and .+-.300 grams in the radial direction.
Astatic levers are not capable of providing supporting force with this degree of precision, particularly in the axial direction. One problem is that an astatic lever tends to sag itself, as indicated by the dashed lines in FIGS. 1 and 2, thus the counterweight moves and the components of the vector W.sub.2 or W.sub.3 change. An astatic lever providing a force of 50 kilograms is accurate at best to within .+-.200 grams. Another problem is that astatic levers do not compensate for non-weight effects such as inertia and wind loading.
Accordingly, the designs of some recent telescopes, such as the 3.58-meter New Technology Telescope at the European Southern Observatory in Chile and the proposed 300-inch telescope being studied by the University of Texas, employ motor-driven actuators rather than astatic levers, particularly for support in the axial direction. The motors that drive the actuators are controlled by a computer. The computer is provided with data indicating, for each telescope attitude, the exact axial supporting force required at each point. Each supporting actuator is equipped with a sensor such as a load cell for measuring the force actually applied. If the applied force deviates from the required force, the computer controls the motor so as to correct the force.
The actuators used in the aforementioned New Technology Telescope in Chile provide support only in the axial direction. They are not suitable for offering radial support, which in this telescope is provided by astatic levers at the perimeter of the mirror. This arrangement is adequate due to the comparatively small diameter of the mirror.
The actuators considered in the design of the aforementioned 300-inch telescope comprise an astatic lever for radial support, a motor-driven mechanism for axial support, and a load cell for sensing the force applied in the axial direction. The load cell is located at an intermediate point in the actuator, behind the motor. A problem with this design is that the load cell is unable to sense the correct force applied to the mirror, because part of the mirror's weight load is carried through the actuator mounting to the mirror cell and it is not carried to the load cell. It is difficult to correct for this effect because the proportion of the load diverted in this way varies complicatedly depending on the telescope attitude and circumstance temperature. A further problem is that if the mirror shifts relative to the mirror cell, the counterweight of the astatic lever begins to exert supporting force in the axial direction as well as the radial direction, but the load cell cannot detect this axial component, causing the controlling computer to misjudge the axial force applied. Yet a further problem is that the astatic lever that provides radial support is subject to sag as mentioned previously. Still another problem is that the motor drives the axial supporting mechanism directly, requiring extremely fine degrees of motor control.
Thus neither of the above systems of mirror support are satisfactory for an extremely large, thin mirror requiring highly accurate support in both the radial and axial directions at a plurality of points.